Method for RF ablation using cooled electrode

ABSTRACT

A catheter for radio frequency ablation with a cooled electrode for use in tissue having a surface comprising an elongate member having proximal and distal extremities. A metal conducting electrode secured to the distal extremity of the elongate member and having a chamber therein. A conductor extends through the elongate member from the proximal to the distal extremity for supplying radio frequency energy to the electrode. The elongate member has a lumen in the distal extremity which is in communication with the chamber. A coolant is disposed in the chamber and in contact with the electrode for dissipating heat created in the electrode by the application of radio frequency energy thereto.

This is a division of application Ser. No. 07/983,732, filed Dec. 1,1992, now U.S. Pat. No. 5,348,554.

BACKGROUND OF THE INVENTION

This invention relates to a catheter for radio frequency (RF) ablationwhich is provided with a cooled electrode and method.

Catheters for RF ablation have heretofore been provided. However,difficulties have been encountered with such catheters in that it hasbeen difficult to achieve lesions of sufficient size. Increasing the RFpower to the catheter in an attempt to increase the size of the lesionshas caused degradation of the blood in the region where ablation istaking place. Such blood degradation has caused products of thedegradation to be deposited on the electrode surface greatly increasingthe impedance. In addition, it has been found that increased powerlevels create undesirable heating of the blood which can create bloodclots. There is, therefore, a need for a new and improved catheter forRF ablation which overcomes these disadvantages.

SUMMARY OF THE INVENTION

In general, it is an object of the present invention to provide acatheter for RF ablation which is provided with a cooled electrode and amethod to make possible the formation of large lesions.

Another object of the invention is to provide a catheter and method ofthe above character in which the catheter is provided with a conductingelectrode having a chamber therein and in which a cooling is provided inthe chamber.

Another object of the invention is to provide a catheter and a method ofthe above character in which the cooling liquid in the cavity ismaintained at a pressure which is substantially equal to the pressure ofthe blood of the chamber in the heart in which the catheter isdisclosed.

Another object of the invention is to provide a catheter and a method ofthe above character in which a pump is provided for introducing thecooling liquid into the catheter and a separate pump is provided forwithdrawing the liquid from the catheter.

Another object of the invention is to provide a catheter and method bywhich lesions or a necrosis can be formed at various depths underlyingthe electrode with destroyed the surface contacted by the electrode.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiment is set forth indetail in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a catheter for use in radiofrequency ablation with a cooled electrode incorporating the presentinvention and showing the same being schematically connected to apumping system for supplying and withdrawing cooling liquid from the tipof the catheter.

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1.

FIG. 3 is a graph showing the positive and negative pressure at thecooled tip is zero at or near the blood pressure range.

FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 5showing another embodiment of a cooled tip incorporating the presentinvention.

FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 4.

FIG. 6 is a side elevational view of another embodiment of a catheterfor use in radio frequency ablation with a cooled electrodeincorporating the present invention showing the same schematicallyconnected to a pumping system for supplying a cooling liquid to the tipof the catheter.

FIG. 7 is a partial side elevational view of the distal extremity of thecatheter shown in FIG. 6.

FIG. 8 is a side elevational view similar to FIG. 7 but showing how thetip rotated through 90°.

FIG. 9 is a side elevational view in cross-sectional of anotherembodiment of a catheter incorporating the present invention taken alongthe line 9--9 of FIG. 10.

FIG. 10 is an end elevational view looking along the line 10--10 of FIG.9.

FIG. 11 is another embodiment of a catheter incorporating the presentinvention utilizing a passive wick for withdrawing energy from the tipelectrode of the catheter.

FIG. 12 is a cross-sectional view taken along the line 12--12 of FIG.11.

FIG. 13 is a cross-sectional view of the distal extremity with anothercatheter incorporating the present invention.

FIG. 14 is a graph showing the temperatures which are encountered intissue during an ablation procedure.

FIGS. 15-18 show graphs showing isothermal curves comparing cooled anduncooled electrodes and the effect on tissue during ablation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the present invention embodies a catheter for radiofrequency ablation with a cooled electrode for use in a heart having awall forming at least one chamber with blood therein. The catheter iscomprised of a flexible elongate member having proximal and distalextremities. An electrode is mounted on the distal extremity of theflexible elongate member and has a cavity therein. Means is providedwhich extends through the flexible elongate member from the proximal tothe distal extremity for supplying radio frequency energy to the tipelectrode. The flexible elongate member is provided with a first lumentherein extending from the proximal extremity to the distal extremityand being in communication with the electrode. Means is provided forintroducing a cooling liquid into the lumen. The means for introducingthe cooling liquid into the lumen includes means for adjusting thepressure of the liquid in the lumen at the electrode so that itapproximates the pressure of the blood in the chamber of the heart inwhich the distal extremity is disposed.

More in particular, the catheter 11 for RF ablation with a cooledelectrode incorporating the present invention consists of a flexibleelongate member 12 having proximal and distal extremities 13 and 14. Ahollow tip conducting electrode 16 is mounted on the distal extremity 14and is provided with an internal cavity 17. The flexibly elongate member12 is formed of a suitable plastic such as a polyurethane. It isdesirable that the plastic utilized be kink-resistant. In order toprovide additional kink-resistance, braid 21 of a suitable material maybe provided within the flexible elongate member 12 during extrusion ofthe same to reinforce the elongate member and to provide additionalkink-resistance. The braid 21 can be formed of a suitable material suchas Nylon or Kevlar. The braid 21 can be provided at the distal extremityof the elongate member as shown in FIG. 1 or, if desired, can extend theentire length of the elongate member 12.

The hollow tip electrode 16 can-be formed of a suitable material such asstainless steel and can have a wall thickness ranging from 0.003 to0.004 inches. The elongate member 12 can be provided in suitable sizesas, for example, to provide catheters from 3 to 7 French in size. Theelectrode 16 has a generally hemispherical configuration and can besecured to the distal extremity of the elongate member 12 by suitablemeans such as an adhesive (not shown). The elongate member 12 isprovided with first and second relatively large liquid carrying lumens26 and 27 which extend from the proximal extremity 13 to the distalextremity 14 and are in communication with the cavity 17 of the hollowtip electrode 16. As shown, lumens 26 and 27 can be crescent-shaped incross-section. A central lumen 28 is also provided which extends fromthe proximal extremity 13 to the distal extremity 14 of the elongatemember 12. A plurality of additional lumens 29 are provided which arespaced circumferentially around the crescent-shaped lumens 26 and 27.The central lumen 28 carries a conductor 31 for supplying radiofrequency energy to the electrode 16. The conductor 31 also serves tosecure the tip electrode 16 so that it remains secured to the distalextremity 14 of the elongate member 12.

Means is provided for steering the distal extremity of the catheter 11and is of the type described in co-pending application, Ser. No.07/793,858, filed Nov. 18, 1991. As described therein, it includessteering wires 33, 34 and 36 which are provided in the lumens 29 andwhich are spaced-apart circumferentially around the central lumen 28 ofthe elongate member 12. The steering wires 33, 34 and 36 together withthe ground return 37 extend to the proximal extremity and are connectedto a male connector 41 which is provided at the proximal extremity ofthe elongate member 12.

As shown, the catheter 11 can be provided with additional radiofrequency electrodes as, for example, electrodes 41 and 42 which areformed as spaced-apart bands provided on the exterior of the distalextremity 14 of the elongate member 12 and in relatively close proximityto the hollow tip electrode 16. Such electrodes 41 and 42 can beconnected by conductors 43 and 44 extending through lumens 29 to theproximal extremity and connected into the connector 41.

The proximal extremity 13 of the elongate member 12 is provided with afitting 51 into which a tubular member 52 and which another tubularmember 53 extends. Tubular member 52 is in communication with the lumen27 and the tubular member 53 is in communication with the lumen 28. Thetubular members 52 and 53 are provided with Luer locks 56 and 57 of aconventional type.

Means is connected to the fitting 56 for introducing a cooling liquidinto the lumen 27 to cause the same to pass through the lumen to thedistal extremity into the cavity 17. Means is secured to the fitting 57for withdrawing the cooling liquid from the cavity 17 so that thepressure of the liquid in the cavity 17 approximates the pressure of theblood in the chamber of the heart in which a catheter is disposed. Thismeans consists of a tank 61 which is provided with a cooled salinesolution 62 therein having a temperature ranging from 5° to 10°Centigrade. It should be appreciated other liquids other than a salinesolution can be utilized, if desired. The means provided for supplyingthe saline cooling solution to the fitting 56 consists of a pump 66which is connected by a tubular member 67 into the saline coolingsolution 62 in the tank 61 and delivers the same through tubular member68 which is connected to the fitting 56 to provide the cooled salinesolution at a predetermined pressure P1 as measured by the pressuregauge 69. Thus, as the pump 66 is operated, the cooled saline solutionis introduced into the lumen 27 and into the cavity 17. In order toreduce the pressure of the cooled liquid in the cavity 17 and tomaintain the pressure in the cavity 17 so that it is substantially equalto the pressure of the blood in the chamber of the heart in which thecatheter is disposed, a pump 71 is provided for withdrawing the cooledliquid from the cavity 17 through the lumen 28. The pump 71 is connectedby a tubular member 72 to the fitting 57 and supplies a negativepressure P2 to the flow passage 28 which is measured by the pressuregauge 73. The pump 71 returns the cooled liquid withdrawn from thecavity 17 through a tubular member 74 into the tank 61 so that it can becooled and reused.

The catheter 11 is adapted to be connected to radio frequency powersupply and controller 76 which is connected by cable 77 to a femaleconnector 78 which is adapted to receive the male connector 32. Theradio frequency power supply 76 can be of a conventional type. Thecontroller incorporates as a part thereof apparatus which is utilizedfor steering the distal extremity of the catheter, of the type describedin co-pending application, Ser. No. 07/793,858, filed Nov. 18, 1991.

Operation and use of the catheter for RF ablation with the cooled tipand the method for using the same can now be briefly described asfollows. Let it be assumed that it is desired to introduce radiofrequency energy into the wall forming a chamber of the heart to causeablation of the myocardium. Also let it be assumed that the catheter isintroduced into the chamber of a heart in a human being in aconventional manner. By utilizing the controller 76, the distalextremity 14 is steered so that the tip electrode 16 is moved intocontact with the myocardium. The high frequency energy can then besupplied from the RF power supply 76 to the tip 16 through the conductor31. Prior to the delivery of such radio frequency energy or at the sametime, the pumps 69 and 73 are placed in operation so that a cooledsaline solution is being introduced into the lumen 27 and into thecavity 17 of the electrode 16 to cool the electrode 16 during the timethat radio frequency energy is being applied to the same. In order tokeep the pressure in the lumen 27 at a relatively low value and so thatthe pressure in the cavity 17 is substantially the same as the pressureof the blood in the chamber of the heart in which the catheter isdisposed, the pump 73 creates a negative pressure to withdraw the cooledsaline solution through the passage or lumen 28 and discharges the sameinto the tank 61. The operator by observing the gauges 69 and 73 canoperate the pumps 66 and 71 in a such a manner so that the pressure inthe cavity 17 is substantially the same as the pressure of the bloodpool surrounding the cavity in which the catheter tip is disposed. Thedesired pressures P1 and P2 on the gauges 69 and 73 can be ascertainedby first operating the pumps 66 and 71 with the catheter 11 outside ofthe body and measuring the pressure in the chamber 17 and then observingthe pressures on the gauges 69 and 73 when the proper pressure ispresent in the cavity 17. Thus, when the catheter 11 is introduced intothe body and into the heart, the desired pressure in the cavity 17 canbe achieved merely by duplicating the readings on the gauges 69 and 73.If desired, a pressure transducer (not shown) can be provided within thecavity 17 and connected through electric conductors (not shown)extending to the proximal extremity of the catheter where the pressurecan be read on an appropriate instrument (not shown).

By maintaining the pressure of the saline solution in the cavity 17, ator near the pressure of the blood in which the distal extremity 14 ofthe catheter 11 is disposed, there is a minimal tendency for leakage ofthe saline solution from the cavity 17 of the catheter 1. This isreadily accomplished even though there is a blood pressure change fromsystolic to diastolic as the heart is pumping blood during the time thatan ablation procedure is being performed. This is illustrated in FIG. 3in which the blood pressure range between systolic and diastolic isshown as ranging from 60-200 millimeters of mercury as approximately 2psi. The pressure changes of the cooled liquid in the lumens 27 and 28is shown by the curve 81. The inlet pressure provided by the pump 66 isP1 as shown in FIG. 3. The pressure drops in the lumen 27 because oflosses in the lumen. Because of the negative pressure P2 created by pump71 at the outlet of the lumen 28, the pressure continues to drop. Byappropriate adjustment of the pressures P1 and P2 to overcome lumenlosses, the pressure in the cavity 17 of the tip electrode 16 can beadjusted so that it approximates the pressure of the blood in thechamber in which the tip 16 is disposed. This is represented by thecurve 81 in FIG. 3 which crosses through the blood pressure range at thetip lumen as indicated at 82. A typical example is shows in FIG. 3 witha catheter 150 centimeters in length and having lumens 0.010" indiameter. A pressure for the cooling liquid at the tip was obtainedutilizing a positive input pressure P1 of 40 psi and a negative outletpressure P2 of negative 40 psi.

It has been found that with a catheter of the present inventionutilizing the method of the present invention it has been possible toachieve lesions of the desired depth of 1/2 to 1 centimeter and asimilar width in the myocardium utilizing 5 to 50 watts of power.

In addition to controlling the pressure in the cavity 17, it still maybe desirable to measure the temperature of the tip electrode 16. Thiscan be accomplished by mounting a thermocouple 83 in close proximity tothe tip 16 and by bringing out leads 84 from the thermocouple through anadditional lumen (not shown) in the elongate member 12 and bringing theleads out to the proximal extremity and connecting them intoinstrumentation 85 coupled to the controller 76 to make the temperaturereading and control the application of RF energy by the controller inaccordance with the temperature reading. This will provide stilladditional input to the physician or surgeon performing the ablationprocedure to ensure that the cooling is adequate and to see thatexcessive temperature is not reached during the ablation procedure.

In order to enhance the cooling of a hollow tip electrode 16 such asshown in FIG. 1, there is provided a modified tip electrode 86 in FIGS.4 and 5. As shown therein, the electrode 86 which can be formed of asuitable conducting material is provided with a plurality of radiallyextending fins 87 which extend inwardly from the cylindrical wall 88 ofthe tip 86 and extend to the distal extremity of the tip as shownparticularly in FIG. 4. The fins 87 terminate short of the longitudinalaxis of the tip 86 to provide a cylindrical space 89 at the extremitiesof the fins 87. Thus, it can be seen as a cooled saline solution entersthe tip 86 in the same manner as saline solution is introduced in thetip 16, the additional cooling surfaces of the fins 87 will causeadditional heat transfer from the electrode 86 to the cooling electrode86. This increased heat transfer from the tip to the cooled salinesolution makes possible less flow of the cooled saline solution or theapplication of additional RF energy to the electrode during the ablationprocedure which is being accomplished.

Another embodiment of the catheter for radio frequency ablation with acooled electrode is shown in FIGS. 6-8. The catheter 91 consists of aflexible elongate member having proximal and distal extremities 13 and14. The catheter 92 is of a suitable length as for example 150centimeters and is provided with a flow passage 96. The distal extremity94 is provided with a portion 92a of reduced diameter and a flow passage97 therein which is in communication with the passage 96. The flexibleelongate member 92 is also provided with the tapered portion 92b whichforms a transition between the distal extremity 94 and the portion 92aof reduced diameter. The passage 97 is in communication with chamber 98provided within a cup-shaped electrode 99 formed of a suitableconductive material such as silver which serves as a hollow tipconducting electrode. The cup-shaped electrode 99 has a proximalextremity 101 secured to the distal extremity 94 of the flexibleelongate member 92 by suitable means such as an adhesive 102. Theelectrode 99 is provided with a hemispherical distal extremity 103having a plurality of spaced-apart holes 106. In addition, the electrode99 is provided with a pair of moon-shaped spaced-apart slots 107 and 108as shown in FIGS. 7 and 8 which are inclined proximally to extend at anangle of approximately 45° with respect to the axis of the tip 99.

Means is provided for supplying a cooling liquid to the chamber 98, inthe electrode 99 and consists of a vessel 111 having a cooled salinesolution therein which can be replenished when desired. Means isprovided for supplying the cooled saline solution from the tank orvessel 111 to the passage 96 of the catheter 11 and consists of an inletpipe 112 which is connected to a pump 113. The pump 113 is connectedthrough a stop cock 114 which is provided with a handle 116 for movingthe same between open and closed positions to provide a three-way valve.The stop cock 114 is connected to flexible piping 117 connected to afitting 118 secured to the proximal extremity 93 of the flexibleelongate member 92.

In addition means is provided for supplying radio frequency energy tothe tip 99 and consists of a radio frequency power supply 121 whichprovided with a power supply cord 122 connected to a connector 123. Theconnector 123 is connected to another connector 124 which is connectedto a cord 126 that is connected into the fitting 118. The cord isconducted by an insulated wire 127 to the electrode 99 by solder 128.

Operation and use of the cooled tip catheter as shown in FIGS. 6 through8 may now be described as follows. The one-way pump 113 serves to pumpthe cooled saline solution from the vessel 111 and supplies it underpressure through the passage 96, through the passage 97 and into thechamber 98. The cooled saline solution rather than being recirculated asin the embodiment shown in FIG. 1 is discharged from the electrode 99 byjets 131 of cooling liquid passing from the electrode 99 after thecooled liquid has come into contact with the electrode. Thus, it can beseen that when the tip 99 is in contact with the wall of the heart, thejets will permeate the interface between the tip 99 and the wall of theheart to produce additional cooling. Although the cooled saline solutionmay have been warmed slightly by the temperature of the tip 99, the jets131 will still be cooler than the surrounding blood and thereforeprovide additional cooling at the interface to help prevent coagulationof the blood at the interface between the tip 99 and the wall of theheart. In addition to the jets 131, they will be additional jetsindicated by the arrows 132 directed proximally from the slots 107 and108 which will create a force that urges the electrode 99 into contactwith the wall of the heart. In other words, jets 132 will create acounter force to the jets 131 to cause the electrode tip 99 to remain incontact with the wall of the heart.

From the foregoing it can be seen that for a given size catheter it ispossible to provide a much larger lumen extending to the distalextremity 94 because only one lumen is required for the one-way flow ofthe cooled saline solution. This increased flow rate of the salinesolution makes it possible to increase the radio frequency powerdelivered to the tip electrode 99 to make it possible to create largerlesions in the wall of the heart when desired. The introduction of thesaline solution into the blood is not objectional because it is alreadydone in a number of other medical procedures.

The three-way valve or stop cock 114 provided makes it possible to drainany air out of the catheter 91 to insure that no air bubbles will bepumped by the pump 113 when pumping the cooled saline solution into thecatheter 91. As soon as the air has been exhausted from the system, thehandle 116 can be turned so that the cooled saline solution is suppliedto the flow passage 96.

By providing the catheter 91 with a cooled ablation electrode which canaccommodate more radio frequency energy, it is possible to performablation procedures other than in the heart. For example, it can beutilized to treat certain tumors. It also can be utilized inelectrocautery and electrosurgery which may make it possible to leavethe surface intact while treating the tissue underlying the surfacewithout damaging the bonding of the surface to the tissue. Thus, intreating an organ through a vein or arterial wall, it is possible to dothis while still preserving the vein or arterial wall without damagingthe lining of the wall. This can be readily accomplished by the cooledtip with the saline solution flowing from the same as shown in FIG. 4 toprevent damage to the wall. Such procedures are particularly applicablefor gall bladder, urology, and gynecology.

In order to prevent blood from entering into the holes 106 and the slots107 and 108 during the introduction of the catheter 91 into the vesselof the patient, it may be desirable to have the cooled saline solutionunder a slight positive pressure as the catheter 91 is introduced intothe passage 96 so that saline solution will be flowing out of the holes106 and the slots 107 and 108. Another alternative would be to apply thecooling saline solution through the passage 96 before RF energy isapplied to the tip 99. This will help to ensure that any blood withinthe catheter will be forced out into the blood pool in which thecatheter is disposed. This will prevent blood from becoming coagulatedwithin the small holes 106 or the slots 107 and 108 when RF energy isapplied and the tip electrode 99 is heated.

Another embodiment of a catheter incorporating the present inventionmaking it more difficult for the blood to enter into the interior of thecatheter is shown in FIGS. 9-10. As shown therein, the catheter 136consists of a flexible elongate tubular member 137 which is providedwith a flow passage 138. A tip electrode 141 formed of a suitableconducting material is adhered to the distal extremity 142 of thetubular member 137 by suitable means such as adhesive 143. A tubularinsert 144 of a suitable material such as plastic is provided with aflared proximal extremity 144a secured to the interior wall of theflexible elongate member 137 by suitable means such as an adhesive 146.As can be seen in FIG. 9, the tubular insert 144 is provided with adistal extremity 147 which terminates short of the hemispherical portionof the tip electrode 141 to provide a space or chamber 148 within theelectrode 141. The tubular insert 144 is provided with a flow passage149 which opens through the distal extremity 147 and which is incommunication with the chamber 148. The flow passage 149 is also incommunication with the flow passage 138 in the flexible elongate member137. A pair of diametrically opposed holes 151 are provided in thedistal extremity 142 of the flexible elongate tubular member 137. Valvemeans in the form of a cylindrical valve sleeve 153 formed of a suitableelastomeric material is disposed in recess 154 provided in the distalextremity 142 of the flexible elongate tubular member 137. The valvesleeve 153 is provided with annular reinforcing ribs which extendcircumferentially around the sleeve 153. Slits 157 extendinglongitudinally of the tubular insert 144 are provided which overlie theholes 151 and serve to form leaflets 153a and 153b yieldably retained ina sealed position to close the slit 157.

It can be seen that when a cooled saline solution is introduced throughthe passage 138 in the catheter 136, the solution will pass through thepassage 149 as indicated by the arrows 157 into the chamber 148 where itwill cool the tip 141. After its cooling function has been performed,the slightly heated cooled liquid will pass proximally as indicated bythe arrows 157 and be discharged into the blood pool under positivepressure through the holes 151 and through the slits 157 by urgingoutwardly the leaflets 153a and 153b provided in the valve sleeve 153.This will permit additional cooled saline solution to be introduced intothe tip electrode 141 to continue cooling of the electrode. Thus, it canbe seen that such a valve sleeve 153 permits the use of a cooled salinesolution while preventing blood surrounding the catheter 136 fromentering into the interior of the catheter when a saline solution is notbeing supplied to the catheter 136.

In certain applications, it may be possible to forego the use of activecooling as for example as by the providing of the cooled saline solutionas hereinbefore described and to rely upon passive cooling for the tipof the catheter while it is being utilized for RF ablation. Such acatheter 171 is shown in FIGS. 11 and 12 and consists of a flexibleelongate tubular member 172 formed of the plastic hereinbefore describedwhich is provided with a flow passage 173 extending therethrough. Thetubular member 172 is connected to a cylindrical metal sleeve 176 formedof a suitable conducting material such as silver. The sleeve can have asuitable length as for example from 2 to 5 centimeters. The sleeve 176is provided with a bore 177 extending therethrough. The other end of thesleeve 176 is connected to another tubular member 179 formed of asuitable insulating material such as plastic. The tubular member 179 isprovided with a large central lumen 181 and a plurality of additionallumens 181 through 187 which are spacial circumferentially around thelumen 181. The lumens 182, 184, and 185 have elements 191, 192, and 193extending therethrough which are connected to the distal extremity ofthe tubular member 179. These elements 191, 192 and 193 are formed of amaterial such as Nitinol having a negative coefficient of expansion. Aground return conductor 194 is provided in the lumen 183. A conductor196 is provided in the lumen 186 and is connected to a hemispherical tipelectrode 197 formed of a conducting material and secured to the distalextremity of the tubular member 179 by suitable means such as anadhesive (not shown).

Passive heat conduction means 201 is provided within the distalextremity of the catheter 171 and consists of a suitable fibrousmaterial such cotton fibers which have been impregnated with a heatabsorbing fluid as for example water or a saline solution. This heatconducting material 201 extends from the distal extremity of the tubularmember 172 through the passage 177 in the metal sleeve 176 and is inintimate contact with the metal sleeve 176. The material 201 alsoextends through the lumen 181 provided in the tubular member 179 andinto the interior of the tip electrode 197. As can be seen from FIG. 11,the various conductors and elements hereinbefore described in the lumens182 through 186 extend through the passive heat conducting material 201and pass through an adhesive 206 then through the passage 173 to theproximal extremity of the catheter 181 where they are connected toappropriate controls of the type hereinbefore described.

In use of the catheter 171 as shown in FIGS. 11 and 12, the applicationof radio frequency energy to the electrode 197 heats the electrode 197to cause the liquid within the passive heat conducting means 201 to heatup and to travel by convection towards the cooler region of the passiveheat conductive means 201. This in turn will cause the cooler liquid tocirculate and take its place. The liquid which has been heated will movethrough the wick-like heat conductive material 201 and will come intocontact with the metal sleeve 176 which will cause cooling to occur byhaving heat pass therethrough into the blood passing the sleeve 176. Thecooled liquid will then return to the tip to continue the convectiveflow as herein before described.

Additionally, if additional heat dissipation is desired, a constructionsuch as that shown in FIG. 13 can be used. Radially extending heatconducting fins 211 of metal are either soldered on or formed integralwith the sleeve 176 on the outer surface thereof. By providing the fins211, additional heat dissipating surface area is provided whichincreases the capabilities for dissipating heat into the bloodcirculating around the sleeve 176 and the fins 211.

The functioning of the catheters hereinbefore described in conjunctionwith ablation by the use of radio frequency energy can be more clearlyunderstood by reference to FIG. 14. FIG. 14 is a graph which along thehorizontal axis shows the depth of the lesion created during theablation procedure in millimeters with respect to temperature in degreescentigrade as shown by the vertical axis of the graph. "0" on the graphis equivalent to the surface of the tip of the electrode which is incontact with the tissue. Going to the right of the graph as shown inFIG. 14, the depth into the tissue increases. Three curves A, B, and Care shown in the graph for three different power levels of radiofrequency energy being delivered into the tissue. The temperature on thegraph goes to 100° C. The 100° C. has been shown because it isconsidered to be an upper limit for temperature or slightly less thanthat because at approximately 90° C. blood begins to boil and coagulateon the electrode tip greatly increasing its impedance and comprising itsability to create lesions. Thus, it is desirable to have the temperatureof the electrode or tip remain below 90° C. if possible. At 50° C. aline 216 has been shown on the graph because this is the temperaturebelow which necrosis of the myocardial as well as connective tissue willcease.

Curve A shown in FIG. 14 is divided into three segments A1, A2, and A3.The broken line segment A2 represents a continuation of the exponentialcurve A3 when no cooling applied to the electrode. Thus it can be seenfrom the power level of 5 watts represented by the curve A that from thetip temperature of 80° C. shown at the commencement of the curve, thetemperature decreases exponentially as the distance from the surface ofthe tissue increases. As shown, the curve A3 crosses the 50° C. necrosisboundary represented by the line 216 at a depth of 5 millimeters. Thelesion created would have a depth of approximately 5 millimeters asrepresented by the distance d1. Further ablation would stop at thispower level. If the tip electrode being supplied with the power levelrepresented by the curve A is actively cooled in a manner hereinbeforedescribed, the tip electrode temperature drops to a much lower level, asfor example 35° C. as represented by the curve A1 at the tip of skininterface at 0 millimeters in distance. Since this temperature is belowthe necrosis temperature, ablation will not begin to occur until adistance of d2 at the point where the curve A2 crosses the necrosis lineat 50° C., as for example a depth of 3 millimeters from the surface.Necrosis will occur at a depth from 3 millimeters to 5 millimeters asrepresented by the distance d3. Such a cooled ablation procedure is veryadvantageous because it permits necrosis to occur below the contactsurface without destroying the contact surface and the tissueimmediately underlying the same. This is particularly desirable inapplications, for example in the heart, in which it is desired to ablatecertain tissues to destroy sites circuits which are causing arrhythmiasin the heart without destroying the surface lining of the heart.

The curve B represents what occurs with and without cooling of theelectrode tip at a higher power level for example, 10 watts of radiofrequency energy for causing I² R heating inside the tissue. Segment B2of curve B represents a continuation of the exponential curve of theSegment B3. As can be seen, the temperature at the tip-skin interfaceapproaches 100° C. which is very objectionable because that is atemperature which boiling of the blood and coagulation of the blood onthe electrode making its impedance high and comprising the ability tocreate lesions. By providing active cooling to the tip electrode, thecurve B1 is generated which shows the temperature at the skin-tipinterface drops to approximately 40° C. and causing necrosis to occurfrom the depth of two millimeters as represented by d4 and extending toa depth of approximately 8 millimeters where the curve B3 crosses the50° necrosis line 216 as represented by d5. Thus it can be seen that itis possible to provide a much deeper and larger lesion using the higherpower level without reaching an undesirable high temperature which couldcause coagulation of the blood on the tip of the electrode. As shown, itis still possible to commence the formation at the lesion below thesurface so that the surface need not be destroyed thus facilitatingearly recovery by the patient from a treatment in accordance with thepresent invention.

Curve C represents a still higher power level, as for example 40 wattsin which the curve is represented by segments C1, C2, and C3. The brokenline segment C2 which is a continuation of the exponential curve C3shows that the temperature at the electrode skin interface far exceedsthe 100° C. and would be unusable except with active cooling provided inaccordance with the present invention. With active cooling, it can beseen that the temperature at the skin electrode interface approaches 80°C. and gradually increases and approaches near 95° and then drops offexponentially to cross the necrosis line 216 at a distance of 15millimeters from the surface of the skin represented by the distance d6.In view of the fact that the starting temperature is above the 50°necrosis line 216, necrosis will occur from the surface of the skin tothe 15 millimeter depth to provide large and deep lesions.

The results which are reflected in the graph in FIG. 14 are alsoreinforced by the thermal contour maps shown in FIGS. 16-18, which showfor cooled electrodes that the higher temperatures are only reached atdepths which are distant from the electrode skin interface.

FIGS. 15, 16, 17 and 18 are graphs which were derived from a computersimulation utilizing the finite element analysis program ANSYS. Thegraphs show computer generated isothermal curves showing thetemperatures which are reached at the electrode 16 at the tip of thedistal extremity of the flexible elongate member 12 of the catheter 11and at different depths in the tissue from the electrode-tissueinterference for different conditions. FIG. 15 represents the situationwhere 10 watts of power are applied to the electrode 16 with nochilling. FIG. 16 is for the same 10 watts of power applied to theelectrode 16 with cooling in the form of 20 ccs per minute of a salinesolution delivered to the electrode at 5° C. FIG. 17 is for a situationwhere 40 watts of power is applied to the electrode 16 without chillingwhereas FIG. 18 is for the situation where the same 40 watts of powerare applied to the electrode with cooling being applied to the electrodeby 20 ccs per minute of a saline solution at a temperature of 5° C. Thegraphs or diagrams shown in FIGS. 16-18 only represent one-half of thetemperature profiles or contours extending radially away from thelongitudinal axis of the distal extremity of the flexible elongatemember 12 of the electrode tip 16.

In the graphs shown in FIGS. 15-18 it is assumed that the electrode tip16 is in contact with tissue such as in contact with the myocardium ofthe human heart. The myocardium is identified as 231 with the blood inthe heart being identified as 232. As can be seen from the graphs inFIGS. 15 through 18 the isothermal curves are identified by letters ofthe alphabet. The isothermal curves shown in FIG. 15 represent thefollowing temperatures at degrees centigrade for an electrode 16 withoutcooling.

A=37.811° centigrade

B=39.433° centigrade

C=41.056° centigrade

D=42.678° centigrade

E=44.300° centigrade

F=45.923° centigrade

G=47.545° centigrade

H=49.167° centigrade

I=50.789° centigrade

J=52.412° centigrade

K=54.034° centigrade

L=55.656° centigrade

M=57.279° centigrade

N=58.901° centigrade

0=60.523° centigrade

P=62.145° centigrade

R=65.390° centigrade

The curve H identified above that represents a temperature ofapproximately 50° C. which is a temperature at which permanent necrosisoccurs in the tissue as for example in the myocardium. In other words,irreversible damage occurs at temperatures higher than this temperature.Below that temperature the edema is temporary and typically reverses.The temperatures represented by the curves I, J, K, L through Rrepresent temperatures above 50° at which necrosis would take place.Thus, because of the temperatures reached, there would be necrosisoccurring from the tip 16 out to a distance or depth represented by thecurve H in FIG. 15. In contrast, by utilizing cooling of the electrodeas shown in FIG. 16, the isothermal curves have the followingtemperatures.

A=19.518° centigrade

B=21.425° centigrade

C=23.333° centigrade

D=25.240° centigrade

E=27.148° centigrade

F=29.055° centigrade

G=30.963° centigrade

H=32.870° centigrade

I=34,778° centigrade

J=36,685° centigrade

K=38.593° centigrade

L=40.500° centigrade

M=42,408° centigrade

N=44,315° centigrade

0=47.228° centigrade

P=48,130° centigrade

R=51.945° centigrade

From the isothermal curves it can be seen that the 50° C. isothermalcurve is spaced from the electrode tip that the temperature only beginsto exceed 49°-50° C. in the vicinity of the isotherm curve R. Thus itcan be seen that a portion of the myocardium immediately adjacent to thetip is saved. In other words, necrosis does not appear in the firstportion of the myocardium because of the cooled tip necrosis does notoccur until a certain distance as for example 1 or 2 millimeters belowthe surface of the myocardium as presented by the curves P and R. Thusit can be seen that the cooling of the ablation tip serves to preservethe surface of the myocardium. At the same time the cooled tip serves toprevent coagulation of the blood which could inhibit power delivery intothe myocardium. In the graph shown in FIG. 17, the isothermal curveshave the following temperatures.

A=40.160° centigrade

B=46.479° centigrade

C=52.798° centigrade

D=59.117° centigrade

E=65,436° centigrade

F=71,756° centigrade

G=78,075° centigrade

H=84,394° centigrade

I=90,713° centigrade

J=97,032° centigrade

K=103,352° centigrade

L=109,671° centigrade

M=1115.990° centigrade

N=122,309° centigrade

0=128,629° centigrade

P=134,943° centigrade

R=147,586° centigrade

Here it can be seen that relatively high temperatures are reached whichare substantially above the 100° C. at which blood coagulates on theelectrode surface. Although a 49° C. isothermal is not shown in FIG. 17,it would be between isothermal curves B and C. Thus, but for thecoagulation of the blood at the electrode tip necrosis should occur to adepth represented by a curve between the curves C and D. This istheoretical only because with temperatures so high blood coagulationwould occur on the tip and greatly interfere with the transfer of powerfrom the tip to the tissue in the myocardium.

In the graph shown in FIG. 18, the isothermal curves have the followingtemperatures.

A=32,980° centigrade

B=38,589° centigrade

C=44,197° centigrade

D=49,806° centigrade

E=55.415° centigrade

F=61.024° centigrade

G=66.633° centigrade

H=72,242° centigrade

I=77.851° centigrade

J=83.460° centigrade

K=89,069° centigrade

L=94,678° centigrade

M=100,386° centigrade

N=105,895° centigrade

0=111,504° centigrade

P=117,113° centigrade

R=128,331° centigrade

From these curves it can be seen that curve D is an isothermal curverepresenting the region at which necrosis would stop. From theisothermal curves in FIG. 18 it can be seen that the temperatures at thesurface of the electrode are substantially below 100° and thereforecoagulation of blood is inhibited from taking place. Thus it is possibleto achieve relatively deep and wide lesions utilizing a cooled ablationelectrode which heretofore was not possible to achieve without cooling.Without the use of cooling for the ablation tip electrode, the amount ofpower which can be supplied to the tip electrode is greatly reducedbecause otherwise the tip electrode temperature rises very rapidlycausing coagulation of blood on the tip which prevents or at leastinhibits the transfer of power from the electrode to the tissue incontact with the electrode.

From the foregoing it can be seen that the provision of a cooledablation electrode has a number of unexpected results. The use of thecooled electrode makes it possible to create necroses well below thesurface being contacted by the electrode. This is particularly desirablefor treating arrhythmias which are created by sub-endocardialarrhythmogenic foci, this makes it possible to spare the endocardium.Thus it is possible to achieve lesions which are several millimetersbelow the tissue surface making it possible to treat tumor cells whichunderlie the skin surface without damaging or breaking the skin surface.This is a great aid in preventing potential infections in a wound. Italso facilitates faster healing for the patient around the necrosiswhich has been created. Thus in accordance with the present invention,it is possible to provide very small lesions for example 2 and 3millimeter diameter lesions in the middle of the myocardium. By theappropriate application of power and the appropriate removal of heatfrom the electrode it is possible to achieve lesions at any desireddepth in tissue without the surface being damaged by the electrode.

Although the present invention is primarily been described in connectionwith ablation of the heart, it should be appreciated that also has otherapplications as for example electrosurgery. It also can be used fortreating tumors underlying the skin as for example breast cancer andprostatic cancer.

In view of the foregoing, it can be seen that there has been provided acatheter which is particularly suitable for radio frequency ablationthat is provided with a cooled electrode and a method for using the samewhich makes possible to ablate the myocardium of the heart withoutcreating undue heating of the blood in the vicinity of the region wherethe ablation is being performed and without causing blood degradation.This can be accomplished by the use of a cooled saline solution which ismaintained at a pressure at the tip of the catheter which issubstantially equal to the pressure of the blood at the tip.

What is claimed is:
 1. A method for creating a lesion in tissue having asurface by the use of an electrode comprising contacting the surfacewith the electrode to form an electrode-surface interface, supplyingradio frequency energy at a predetermined power level to the electrodeto cause heating of the tissue in the vicinity of the surface contactedby the electrode and cooling the electrode so that the temperature inthe tissue at the electrode-surface interface does not reach atemperature in excess of approximately 50° C., said cooling beingaccomplished by supplying a cooling fluid to the electrode during thetime the tissue is being heated and withdrawing the cooling fluid afterit has cooled the electrode.
 2. A method for creating a lesion in tissuehaving a surface by the use of an electrode comprising contacting thesurface with the electrode to form an electrode-surface interface,supplying radio frequency energy at a predetermined power level to theelectrode to cause heating of the tissue in the vicinity of theelectrode-surface interface and cooling the electrode by recirculating aflow of cooling fluid to prevent the temperature of the tissue extendingfrom the electrode-surface interface to a predetermined depth in thetissue from reaching a temperature in excess of approximately 50° C. sothat a necrosis is not formed in the tissue at the electrode-surfaceinterface or in the predetermined depth in the tissue.
 3. A method forperforming an ablation in the myocardium of a heart by utilizing acatheter having a distal extremity with a conductive electrode mountedthereon comprising introducing the catheter into the heart so that theelectrode is in close proximity to and in contact with the myocardium,supplying radio frequency energy to the electrode and supplying acooling liquid under a positive pressure to the electrode for coolingthe electrode during the time that the radio frequency energy is beingsupplied to the electrode and withdrawing the cooling liquid from theelectrode at a negative pressure so that a predetermined pressure ismaintained at the electrode.
 4. A method as in claim 3, together withthe step of adjusting the predetermined pressure at the electrode sothat it is substantially equal to the pressure of the blood in the heartsurrounding to electrode.
 5. A method for creating a lesion in tissuehaving a surface by the use of an electrode comprising contacting thesurface with the electrode to form an electrode-surface interface,supplying radio frequency energy at a predetermined power level to theelectrode to cause heating of the tissue in the vicinity of the surfacecontacted by the electrode and cooling the electrode so that thetemperature in the tissue at the electrode-surface interface does notreach a temperature in excess of approximately 50° C., the step ofcooling the electrode including the step of delivering a cooling liquidto the electrode, and creating jets of the cooling liquid exiting fromthe electrode in a direction extending generally forwardly from theelectrode to impinge upon the electrode-surface interface.
 6. A methodas in claim 5 together with the steps of creating additional jets ofcooling liquid exiting from the electrode in a direction proximal of theelectrode to urge the electrode into contact with the surface.